Method of making a silica containing carbon brush and resulting article



N v, 4, 1958 D. RAMADANOFF 2,859,139

METHOD OF MAKING A SILICA CONTAINING CARBON BRUSH AND RESULTING ARTICLE Filed Dec. 7, 1954 Impregnale Carbonaceous Brush Srock wirh a Solution conlaining a Silicon Compound 'Precipirale Silicon Compound in Siru v 'Decompose Silicon Compound in Silu lo Silica Bake Brush Slock at I000 "C to I600 C Carbonaceoua Br ush containing Silica |n heat frgealed condnion INVENTOR 5 DlMlT R RAM BY Q ATTORNEY OFF 7:) i": ii] Nil-Gi li METHOD OF MAKING A SILICA CONTAINING CARBON BRUSH AND RESULTING ARTICLE Dimiter Ramadanoff, Berea, Ohio, assignor to Union Carbide Corporation, a corporation of New York Application December 7, 1954, Serial No. 473,747

6 Claims. (Cl. 117-228) This invention relates to an improved commutator brush, and more particularly concerns a silica-containing carbon brush, and an improved method of making same.

For some time now silica has been used as a polishing or abrasive material in carbon brushes. In U. S. Patent No. 1,991,487, issued to A. S. Bemis, February 19, 1935, a silica-containing carbon brush is disclosed, which is useful for polishing or abrasive action. Such a brush operates satisfactorily under normal conditions of use. However, brushes manufactured in accordance with the Bemis patent have been found to be unsuited for use in confined spaces containing silicone vapors. Such conditions may arise when silicone resins are used as a coating material on armature windings. In this silicone atmosphere the life of the Bemis brush is considerably shortened. At times its wear is so pronounced that it begins to dust and wear out in a matter of hours. Up to the time of the present invention, no brush has been found to operate in a silicone atmosphere comparable to that of an ordinary carbon brush operating in a normal air atmosphere.

It is, therefore, an important object of the present invention to provide a carbon commutator brush capable of operating satisfactorily in silicone-contaminated air atmospheres and in normal air atmospheres.

Another object of the present invention is to provide an improved method of fabricating a carbon brush to effect efficient operation thereof in silicone-contaminated air atmospheres and in normal air atmospheres.

Yet another object of the invention is to improve upon present day silica-containing carbon brushes so that said brushes may satisfactorily operate in the presence of silicone vapors, yet continue to perform satisfactorily in a normal air atmosphere.

Fig. 1 is a flow diagram illustrating the method of this invention.

Fig. 2 is an isometric view of a carbonaceous brush embodying this invention.

The present invention is an improvement on the behavior of present day silica-containing carbon brushes operating in a silicone atmosphere. It not only imparts a resistance to the deleterious effects produced by a silicone atmosphere, but renders the brush operative in a normal atmosphere to the same degree as a brush having no silica content. This is accomplished by subjecting the silica-containing brush to a relatively high temperature bake. For the brush to operate with maximum life in an atmosphere contaminated with silicone vapors, it must be baked for a suitable period of time at a predetermined temperature falling within a critical temperature range between 1000 C. and 1600 C. A preferred temperature range is between 1400 C. and 1500 C.

The brushes to which it is intended to apply the method of this invention may be either plain carbon brushes, graphitic brushes, or electrographitic brushes, the latter being preferred in the present instance. A typical silica- States Patent "ice containing carbon brush embodying the principles of the invention may contain between 0.3 percent and 1.0 percent silica, preferably between 0.5 percent and 0.8 percent silica.

While the high temperature baking process of the present invention improves the wear characteristics of silicacontaining brushes operating in a silicone atmosphere, the mechanics of this heat treatment are not entirely understood. Up to the present no complete reason has been advanced to explain the improved performance brought about by the high temperature baking action of the invention.

The silica-containing carbon brushes to be treated in accordance with the method of the present invention may be preliminarily prepared in any of several ways. One method of incorporating silica in a carbon brush is described in the aforementioned A. R. Bemis Patent No. 1,991,487. Another method consists in impregnating the brush stock with a solution of silicone resin in a solvent, toluene being a preferred solvent in the present instance. The brush is then dried to remove the solvent. Thereafter it is baked at a relatively low temperature between 600 C. and 850 C. to decompose the silicone resin and form silica.

Another method of introducing silica in a carbon brush stock consists in impregnating the brush stock with ethyl silicate, preferably a 10 percent solution of ethyl silicate in ethanol. While still wet, the impregnated brush is placed in a concentrated ammonium hydroxide solution. The effect is to produce a precipitate of silica which is uniformly distributed throughout the brush. The brush is then dried in the same manner described in the preceding example.

The preferred method of introducing silica uniformly throughout the brush composition is to impregnate the brush stock with a colloidal solution of silica in water. Thereafter the impregnated brush is quickly chilled to 0 C. or lower so as to irreversibly precipitate silica in the brush in uniform distribution. The brush may then be dried and baked in the same manner as in the preceding examples.

The above low temperature baking operation may, if so desired, be combined with the high temperature baking process of the present invention into a single baking operation. In such case the baking temperature is then gradually increased up to a temperature between 1000 C. and 1600 C.

Tests of silica-containing carbon brushes which have been baked in the high temperature range specified hereinabove have shown them to be admirably suited for operation in either a normal or uncontaminated air atmosphere or a silicone vapor-containing atmosphere.

As an illustration of the effect of the high temperature bake of silica-containing brushes has on brush life and other physical characteristics, tests were conducted on plain electrographiti brushes. These brushes operated in an air atmosphere with a brush life of 3500 hours per inch. Brushes of this composition were impregnated with a silicone resin and baked at 200 C. and then at 600 C. The brush life of these baked brushes was found to be only 880 hours per inch when operated in an air atmosphere. A subsequent rebake of these brushes at 1400 C. in accordance with the principles embodied in consecutively baked up to 1300 C. and operated in an air atmosphere.

Illustrative of the operation of the brushes of the present invention in a silicone-containing atmosphere, the following tables strikingly show the improvement brought about in brush life when brushes containing silica are baked to a temperature within the range of 1000 C. to 1600" C. Table I pertains to electrographitic brushes in which silica was introduced in accordance with the preferred method of the invention. The brushes were operated in an air atmosphere contaminated with silicone vapors.

Table II applies to electrographitic brushes in which silica was introduced by impregnation with silicone resin. These brushes were operated in an uncontaminated air atmosphere. In Experiment C the brushes were baked in successive steps to 1000 C., 1300 C., 1400" C., 1500 C., 1500 C. and 1600 C. Between each step the brushes were cooled and tested to determine brush life.

Table l Silica Con- Brush Life Baking Temperatures, C. tent in Hours Per Brush, Per- Inch centage 0. 50 1,310 0. 50 5, 550 0.77 14, 000 0.58 20,800 None 2, 785

Table II EXPERIMENT A Brush Life, Baking Temperature, C. Hours Per Inch EXPERIMENT B 985.-.. 3, 400 i 2nn 6, 200 1 200 6,400 1,400 7, 800

EXPERIMENT C (CONSECUTIVE BAKING) From the above it will be seen that brushes having a silica content between 0.3 percent and 1.0 percent, and which are further baked at a relatively high temperature falling between 1000 C. and 1600 C. may be operated with acceptable life in an air atmosphere, and show marked improvement when operating in a silicone vapor atmosphere.

Modifications and variations may he elfected without departing from the scope of the novel concepts of the present invention.

What is claimed is:

l. A method of imparting long wearing life to a carbon brush for operation in a silicone-containing atmosphere, comprising incorporating silica in said brush in an amount between 0.3 percent and 1.0 percent by weight of the brush, and thereafter baking said brush at a temperature between 1000 C. and 1600 C. thus converting the silica into a suitable form for imparting increased wear life to said brush in said atmosphere.

2. A method of imparting long wearing life to a carbon brush for operation in a silicone-containing atmosphere, comprising impregnating the brush with a solution of silicone resin in toluene, drying the brush to remove the toluene, decomposing the silicone resin to silica in an amount between 0.3% and 1.0% silica by baking the brush at a temperature between 600 C. and 850 C., and thereafter rebaking the brush at a temperature between l000 C. and 1600 C.

3. A method of imparting long wearing life to a carbon brush for operation in a silicone-containing atmosphere, comprising impregnating the brush with a solution of ethyl silicate to provide between 0.3% and 1.0% silica, exposing the brush while still wet to concentrated ammonium hydroxide, baking the brush at a temperature between 600 C. and 850 C., and thereafter rebaking said brush at a temperature between 1000 C. and 1600 C.

4. A method of imparting long wearing life to a carbon brush for operation in a silicone-containing atmosphere, comprising impregnating the brush with a colloidal solution of silica in water, chilling the impregnated brush to about 0 C. to uniformly precipitate silica in the brush in an amount between 0.3% and 1.0%, baking the brush at a temperature between 600 C. and 850 C., and thereafter rebaking said brush at a temperature between 1000 C. and 1600 C.

5. A method of imparting long wearing life to a car bon brush for operation in a silicone-containing atmosphere, comprising impregnating the bgush with a colloidal solution of silica in water, chilling the impregnated brush to about 0 C. to uniformly precipitate silica in the brush in an amount between 0.3% and 1.0%, and gradually baking the brush up to a temperature between 1000 C. and 1600 C.

6. A carbonaceous b rggsh for an electrical machine which in iml'iise is required to operate in a silicone-containing atmosphere of a type having a tendency to cause rapid deterioration of carbonaceous brushes when operated therein, said brush being characterized by its resistance to deterioration under such conditions and by its content of between 0.3% and 1% by weight of silica, said silica being the constituent of said brush eifective to impart to it said resistance to deterioration under such conditions, said silica being substantially uniformly dist r ib lled throughout said brush and being in agondition substafinauv'ieenficar-wimmatpfaafiea'"fiifbakifig carbonaceous brush stock impregnated with silica in the temperature range between 1000 C. and 1600 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,480,675 Dirzuweit Jan. 15, 1924 1,729,343 Haas Sept. 24, 1929 1,991,487 Bemis Feb. 19, 1935 2,216,493 Kiefer Oct. 1, 1940 2,790,736 McLaughlin Apr. 30, 1957 

1. A METHOD OF IMPARTING LONG WEARING LIFE TO A CARBON BRUSH FOR OPERATION IN A SILICONE-CONTAINING ATMOSPHERE, COMPRISING INCORPORATING SILICA IN SAID BRUSH IN AN AMOUNT BETWEEN 0.3 PERCENT AND 1.0 PERCENT BY WEIGHT OF THE BRUSH, AND THEREAFTER BAKING SAID BRUSH AT A TEMPERATURE BETWEEN 1000*C. AND 1600*C. THUS CONVERTING THE SILICA INTO A SUITABLE FORM FOR IMPARTING INCREASED WEAR LIFE TO SAID BRUSH IN SAID ATMOSPHERE. 